DDP has stated himself that he personally called Randy and told him to take the Cutter as his move. Randy was according to Page, rather pilled up and incoherent at the time so didn't remember the call and thought he had heat with Page over it.
In other cases it comes down to whether the guy who originates ok's it like Page did...after all this was a guy who won a case over his hand gesture, so if he didn't want the cutter used he could have made life hard for Orton.
It's rare to see out and out "stealing" of moves while a guy is still using it. The F5/FU was perhaps the most obvious "steal" of a move so far as Cena used it in he and Brock's mini feud as a "dig". That he has kept it all these years was perhaps WWE's dig on Brock for leaving as he did and by now it's Cena's move rather than Brock's in the scheme of things.
I guess in the end it comes down to where you are on the totem when the person steals the move.. If someone came in tomorrow and did a Warrior style Gorilla Press then it wouldn't really be stealing it as he isn't there...Page's argument to Orton was "I'm done, take the move". but if someone started using the Pedigree or the RKO while they're still there it would probably be an issue. If someone used the Perfect Plex while Axel was using it, you could see heat from it but it would be based more on it being Curt's move and disrespectful rather than it being Joe's move.
Ultimately there is a finite number of safe moves and variations possible, much like anyone who writes a piece of music now has undoubtedly been influenced by 300 years+ of composition. It's almost impossible to create new, safe moves - anything untried is unlikely to pass due to the safety implications and anything overtly risky will be stopped. The days of creative finishes are gone, you'll likely start seeing more traditional ones coming back as a result so there will be an element of "stealing" from far more recently than previously.
A few problems with your post. The F5 and the FU are two completely different moves. The F5 is a shoulder carry into a twist with a side facebuster/reverse suplex style impact face down. The FU is a shoulder carry into an elevated Fireman's carry toss/side drop onto the back. The only similarity is the shoulder carry setup and the name. However it's no longer called the FU, it's now the Attitude Adjusment and has been for about 4 years now.
DDP didn't invent his cutter either. He took it from Johnny Ace and the AceCrusher. It was then used by DDP and others including Bubba Ray Dudley and variations became the Twist of fate and a major mod combining 2 moves of the cutter and the sitout jawbreaker became the Stone Cold Stunner.
Warrior's gorilla press slam was a 'theft' of a move itself as it was apparently originated or popularized by Gorilla Monsoon hence the name. Also called the military press at various times, was also used by powerhouses like the LOD and the like.
You are right that there has reached a stale level of creative new finishers. The human body can only move in so many ways and be manipulated in so many others and with about 100 years of history in 'professional' wrestling, and the way that audiences and wrestlers have evolved a plateau has been reached. It's now at a point were flash needs to be adjusted to improve and maintain safety. The days of constant expanding of limits and pushing of luck when performing some moves has come to an end. Any further pushing could start resulting in more injuries and possibly more serious health risks for the performers. That's not to say that there still isn't a little room between safety margins and creativity for new moves to come about, but those limits are much more narrowly defined then they have been since the late 80's.
Now I didn't bring this up before since I was waiting to see if anyone else did, but something that has been overlooked is the in match 'steals' that happen often during big moments. Times when during a match, one wrestler will use his opponents finish against him in order to try and show him up by doing it better or add insult to injury by beating him with his own move. This is part of the script of the matches and usually planned out ahead of time, but there have been times when it happened spontaneously and added a little bit of shoot into the match. These momentary use of moves is appropriate and welcome as it adds impact and tension to the match, even if only for a short time. And of course, if one person uses the others finish but there's a kickout, then 100% of the time, the opponent who's move was stolen first will then have to make an attempt to use the first person's move as well.
Personally, I think the idea of one person stealing another's move is foolish. Even if someone has invented a new move, once it's out there, it's available for any one to use. The single biggest and most common modern example is of course the DDT. Invented by Jake the Snake(accidentally when opponent fell onto face, he followed him down and a botched moment became a devastating move) it is now used as a staple in the arsenal of all wrestlers. Everyone uses some kind of variation of it at some point, perhaps not in every match, but everyone is trained in how to do in in various situations. In all honesty though, if there was such a thing as patenting a move to prevent others from using it, EVERY single move would have been patented and their inventors making tons of many from being able to demand a royalty for allowing others to use them.
